1. Field of the Invention
The present invention relates to a hybrid catalyst, more specifically, to a hybrid catalyst which is prepared by mixing a methanol synthesis catalyst with SAPO-type zeolite as a methanol conversion catalyst, and a process for the preparation of hydrocarbons from carbon dioxide by using the hybrid catalyst.
2. Background of the Invention
In general, hydrocarbons are synthesized from carbon oxides such as carbon monoxide or carbon dioxide via hydrogenation by employing hybrid catalysts, that are prepared by mixing a catalyst for the synthesis of methanol, an intermediate in the process of preparing hydrocarbons from the carbon oxides, with a methanol conversion catalyst.
For example, Chang et al. synthesized hydrocarbons via hydrogenation of carbon monoxide by employing Cr/Zn/Al as the methanol synthesis catalyst and HZSM-5 as the methanol conversion catalyst, respectively(see: C. D. Chang et al., J. Catal., 90:84, 1984). Also, Fujimoto et al. reported that hydrocarbons can be made from carbon monoxide by using a hybrid catalyst prepared by mixing a methanol synthesis catalyst such as Pd/SiO2 or Cu/Zn with a methanol conversion catalyst such as HZSM-5 and H-Mordenite(see: K. Fujimoto et al., I and EC Res., 27:920, 1988).
As described above, many efforts have been made to prepare hydrocarbons via hydrogenation of carbon monoxide, while hydrogenation of carbon dioxide has not been successfully accomplished, since the carbon dioxide is a more stable chemical in terms of thermodynamics than carbon monoxide.
Carbon dioxide is one of the most abundant carbon sources and its concentration is continuously increased in the atmosphere, which plays a critical role in causing the greenhouse effect on Earth. For this reason, it is urgently needed to develop a method for converting carbon dioxide to high-valued chemical compounds such as methanol and hydrocarbons, as an approach to overcome the limitation of carbon sources and also improve the air environment on Earth.
Under this circumstance, Fujimoto et al. successfully synthesized hydrocarbons from carbon dioxide by using a hybrid catalyst comprising a methanol synthesis catalyst such as Cu/Zn or Zn/Cr, and a methanol conversion catalyst such as DAY(see: K. Fujimoto et al., Appl. Catal., 31:13, 1987), although the yield of hydrocarbons is very limited(yield=4.1% (w/w)).
Thereafter, Inui et al. synthesized hydrocarbons from carbon dioxide, by using a hybrid catalyst made of Cu/Cr/Zn and HZSM-5 at 50 atm and 320xc2x0 C.(see: T. Inui et al., Appl. Catal., 94:31, 1993), and Fujiwara et al., by using a hybrid catalyst composed of Cu/Zn/Cr and Y zeolite(see: M. Fujiwara et al., Appl. Catal., A:General, 121:133, 1995). However, the prior art catalysts have revealed some shortcomings in the senses that: they are not appropriate to synthesize the higher alkanes, since the reactions are generally terminated to produce ethane only, the yields of hydrocarbons are very limited.
Although numerous researchers have continuously made efforts in order to develop catalysts available for producing hydrocarbons via hydrogenation of carbon monoxide, carbon dioxide, etc., the prior art hybrid catalysts comprising HZSM-5, Y zeolite, etc. as the methanol conversion catalyst provide low yield of hydrocarbons. In addition, only limited kinds of hydrocarbons can be prepared by employing the catalysts and a high pressure of more than 50atm is essentially accompanied by the reaction.
Under these circumstances, there are strong reasons for exploring and developing a novel hybrid catalyst which can produce hydrocarbons in a high yield, while allowing the production of various hydrocarbons.
The present inventors have made an effort to solve the problems of the prior art hybrid catalysts to produce hydrocarbons from carbon dioxide, and they prepared a novel hybrid catalyst by mixing a methanol synthesis catalyst with zeolite SAPO-type which is a methanol conversion catalyst, and discovered that hydrocarbons can be synthesized from carbon dioxide by using the hybrid catalyst in a high yield under a relatively mild condition.
A primary object of the present invention is, therefore, to provide a hybrid catalyst for production of hydrocarbons from carbon dioxide.
The other object of the invention is to provide a process for preparing hydrocarbons from carbon dioxide by using the hybrid catalyst.
A hybrid catalyst for production of hydrocarbons from carbon dioxide of the present invention is prepared by mixing a methanol synthesis catalyst and a methanol conversion catalyst of SAPO-type zeolite. The hybrid catalyst of the invention allows the synthesis of hydrocarbons having a carbon number of more than 2 in a high yield under a low pressure.
The hybrid catalyst of hte invention isexplained in more detail as below.
The present inventors prepared a Cu/ZnO/ZrO2 catalyst as the methanol synthesis catalyst by the conventional coprecipitation method in the art. The Cu/ZnO/ZrO2 catalysts contains: Cu of 10 to 60% (w/w); ZnO, 0 to 30% (w/w); and, Zro2 10 to 90% (w/w), most preferably Cu, 60% (w/w); ZnO, 30% (w/w); and ZrO2, 10% (w/w), respectively. And then, SAPO-5 and SAPO-34, which are SAPO-type zeolites, are employed as the methanol conversion catalyst. The hydrogen ion outside of lattice in the SAPO-5 and SAPO-34 zeolites may be substituted for copper ion, where the content of copper is 0.5 to 2% (w/w), preferably 1 to 2% (w/w). The hybrid catalyst of the present invention may be prepared by mixing a methanol synthesis catalyst such as Cu/ZnO/ZrO2, Cr2O3/ZnO/Al2O3, Pd/SiO2, Cu/ZnO, ZnO/Cr2O3 and Cu/Cr2O3/ZnO catalysts with a methanol conversion catalyst of SAPO-type zeolites which may contain Cu optionally, where the methanol synthesis catalyst and the SAPO-type zeolite are mixed in a ratio of 1:4 to 4: 1(w/w), most preferably 1:1 (w/w).
A process for preparing hydrocarbons from carbon dioxide by using the hybrid catalyst of the invention is illustrated in more detail as followings.
The process for preparing hydrocarbons from carbon dioxide comprises the steps of: reducing a hybrid catalyst by placing the hybrid catalyst prepared by the invention in a high pressure reactor, purging H2/Ar, elevating the temperature of the reactor to reach 250 to 350xc2x0 C. gradually under atmospheric pressure, and increasing the concentration of hydrogen gas to 100%; and, hydrogenising carbon dioxide to give hydrocarbons by pressurizing the reactor to reach 25 to 35atm by purging helium gas to the reactor, purging CO2 and H2, to maintain H2/CO2 of 3 and W/F of 10 to 30 g-cat h/mol, and elevating the temperature of the reactor to reach 350 to 500xc2x0 C.
The products are analyzed by the aid of gas chromatography connected with the said reactor to examine the conversion of carbon dioxide and the yield of hydrocarbons. As a result, it was determined that hydrocarbons having a carbon number of more than 2 were synthesized in a high yield under a relatively low pressure. In particular, hybrid catalysts comprising a methanol conversion catalyst of SAPO-34 and SAPO-5 revealed to have a high selectivity on propane and butane, respectively. Also, it was found that the activity of the hybrid catalyst is increased by substituting copper ion for hydrogen ion outside of the lattice in the said SAPO-type zeolites.